Calcium and phosphate homeostasis in animals and humans is regulated by vitamin D metabolites, and the compound 1.alpha.,25-dihydroxyvitamin D.sub.3 is generally considered as the most active and most important vitamin D-derived regulator of normal calcium and phosphate balance. This natural metabolite and compounds structurally related to it are therefore of great pharmaceutical interest as effective agents for the prevention and treatment of bone diseases and related calcium metabolism disorders. In addition to the natural D.sub.3 metabolites, a number of compounds have been prepared in recent years which, because of their high potency, find use or show very considerable promise as therapeutic agents, among them 1.alpha.-hydroxyvitamin D.sub.3, 1.alpha.-hydroxyvitamin D.sub.2, 1.alpha.,25-dihydroxyvitamin D.sub.2 and certain fluorinated analogs (U.S. Pat. Nos. 3,741,996; 3,907,843; 3,880,894; 4,226,788; 4,358,406). Most of the known active analogs are characterized by the type of sterol side chain as it occurs in vitamin D.sub.3 (i.e. saturated side chain). Known analogs with 22,23-unsaturated side chain are represented by compounds of the vitamin D.sub.2 series (i.e. 22,23-trans-unsaturated with a C-24-methyl substituent), and include, in addition to the compounds named above, 25-hydroxyvitamin D.sub.2 (U.S. Pat. No. 3,585,221) and the 24- and 24,25-dihydroxy derivatives (Jones et al. Arch. Biochem. Biophys. 202, 450 (1980)) and three compounds lacking the 24-methyl substituent (U.S. Pat. No. 3,786,062; Bogoslovskii et al. J. Gen. Chem. USSR, 48(4) 828 (1978); Chem. Abstr. 89, 163848j, 209016s).